The release of chemicals into the atmosphere, often polluting the air is of substantial concern. Thus, in the chemical industry as new products and processes are developed, a key factor is the environmental effect. One means of reducing chemical emissions is to develop solvent-free processes, and to require that chemicals do not evaporate during processing or from the final product.
Traditionally, release coatings have been solvent-borne thin coatings, i.e., dry thickness below about 5 micrometers. For continuous liquid coating techniques, the composition typically has been diluted with a large amount of a solvent that is later removed by evaporation, leaving behind the composition at the desired thickness. The uniformity and thickness of the dried final layer may be difficult to control especially on rough surfaces. The added solvent leads to higher material costs, preparation costs, and solvent removal costs. In addition, the solvents typically used may be hazardous to the environment.
Solvent-borne thin coatings may also be applied by spray processes. Although spray coating may be used to apply a composition to a smooth substrate, it is particularly useful as a method of coating a rough or three-dimensional substrate. A problem associated with conventional spray processes is poor coating efficiency where a substantial amount of the coating composition does not land on the substrate. However, electrostatic spray processes provide a more controlled means of spraying, and thus reduce material loss. Electrospray, a distinct subclass within electrostatic spraying, may be used to apply a thin coating even without a solvent. Typically, electrospray can be used to apply a coating with a thickness from about 0.005 micrometers to about 10 micrometers.
Although the electrospray process is an effective means of applying a thin coating, not every composition can be electrosprayed. The composition must meet certain processing requirements. Among the requirements for electrospray are that the composition be essentially a single phase solution and not a dispersion (solids-in-liquid) or emulsion (liquid-in-liquid), that the composition have sufficient conductivity, and that the composition have a relatively low viscosity.
Although a composition with a conductivity between 10.sup.-7 siemens per meter (S/m) and 10.sup.-1 S/m can be electrosprayed, for thin coatings the droplets preferably are relatively small which requires a conductivity greater than 10.sup.-6 S/m or 1 microsiemens per meter (.mu.S/m). Likewise, because the range of flow rates where a solution can be electrosprayed decreases with increasing conductivity, to obtain flow rates desirable in production the conductivity preferably is held below about 10.sup.-3 S/m (1000 .mu.S/m).
Compositions can be electrosprayed with or without a solvent, provided the composition is either a single phase solution or a non-ionically-stabilized emulsion or dispersion. If the composition is not essentially a single phase solution, the composition may become unstable during the electrospray process. In a single phase solution ("true solution"), each component is completely soluble. Often a solvent must also be added to the composition in order to obtain the requisite solubility. This added solvent, particularly if organic, may present environmental problems if it evaporates during or after processing and is not captured.
When a composition is truly solvent-free, substantially all of the initial components are present in some form in the final cured product. Thin coatings exist which are solvent-cast, but do not fit this definition because the solvent evaporates during processing. For example, although ethanol or methanol can be added to electrosprayable compositions to enhance solubility and conductivity, they evaporate during processing. For some free-radical curing systems, such solvents may interfere with polymerization by serving as chain transfer agents or as inhibitors, and preferably they are removed prior to curing.
Water-based compositions, although sometimes termed solvent-free, typically require large drying ovens, which occupy a sizable portion of manufacturing space and add to the product cost. In addition, often compositions to be electrosprayed are organic, and thus tend to be immiscible with water.
During electrospraying, the sprayhead places charge onto droplets by the principle of electrostatic induction. For inductive charging to work, the conductivity of the spraying composition must be within a specific range.
A solvent can be added to a composition to enhance conductivity. To achieve the desired conductivity range, in addition to containing a conductivity enhancer, i.e., salt, compositions often contain a polar solvent typically considered to be a volatile organic compound (VOC). These volatile organic compounds can be hazardous to the environment.
For electrospray, solvents have been used to increase solution conductivity. For example, EPO Appln No. 92.907947.3 (Mazurek et al.) discloses adding methanol in small quantities to enhance the conductivity of an electrosprayable release coating. However, methanol evaporates during processing, otherwise it may adversely interfere with free-radical polymerization.
U.S. Pat. No. 4,059,444 discloses adding quaternary ammonium salts, which have inorganic anions with relatively low molecular weights, as conductivity enhancers such as sulfate, borate, and iodide, to ink. These conductivity control agents are added at levels of 0.05 to about 1 weight percent to increase the conductivity of electrostatically applied inks.
U.S. Pat. No. 5,364,726 discloses a liquid developer comprising a colorant and a curable liquid vehicle, solid particles containing an initiator which is substantially insoluble, and optionally conductivity enhancing agents such as quaternary ammonium compounds as described in U.S. Pat. No. 4,059,444.
U.S. Pat. No. 4,303,924 discloses adding an oil-soluble salt, such as the mineral acid and organic acid quaternary salts of the Group Va elements, to a curable printing ink containing 0 to 30% of a polar organic solvent. All examples include a polar organic solvent.
To electrospray a thin layer having uniform thickness, each droplet from the electrospray mist preferably has a sufficiently low viscosity to allow for reasonable spreading on the substrate. However, for some applications, it may be desirable to cure individual droplets on the substrate, e.g., slip sheets.
Many release coatings known in the art contain silicones such as polydimethylsiloxane for release properties. Generally, the viscosity of these compositions tends not to be low enough for electrospraying. Thus, solvents have been added to control viscosity. Alternatively, reactive diluents have been added to control viscosity. For example see WO95/23694 (Kidon et al.) and U.S. Pat. No. 4,201,808 (Cully et al.).
Regardless of the method of applying a thin release coating to a substrate, the release coating components preferably do not detrimentally interfere with the final performance of the product. A component preferably does not evaporate or interfere with polymerization or becomes physically trapped in the coating during processing otherwise the component may migrate into the substrate and detrimentally affect the product's performance. Alternatively, an uncured component may later evaporate polluting the environment, or may later contact another surface, rub off, and contaminate that surface. Thus, the need exists for a release coating composition capable of being electrosprayed where substantially all of the components are present in the final product and either co-polymerize with the other components or otherwise become a permanent part of the coating.